1. Field of the Invention
The present invention relates to a low noise amplifier, and more particularly to a low noise amplifier and related method in a wideband communication system.
2. Description of the Prior Art
A low noise amplifier (LNA) is a common device used to filter out the noise of input signals received at the front ends of communication systems. Generally speaking, the LNA is capable of decreasing most of the incoming noise and amplifying a desired signal within a certain frequency range to increase the signal to noise ratio (SNR) of the communication system and improve the quality of received signal as well. Because the desired signal is transmitted via a carrier, the frequency range of the desired signal is supposed to be near the carrier frequency. Therefore, the LNA is designed to magnify an input signal with a frequency close to the carrier frequency and attenuate an input signal with a frequency far away from the carrier frequency through a common gate or common source amplifier with a frequency-dependent loading, such as an inductor connected to a capacitor in parallel.
Please refer to FIG. 1, which is a schematic diagram illustrating the frequency response 10 of a prior art narrow-band LNA. Taking the narrow-band LNA of a wireless communication system for example, the frequency response 10 of the LNA has a center frequency of 2.43 GHz and a bandwidth about 200 MHz. As one can see, an input signal with a frequency equaling 2.43 GHz is allowed to have a maximum magnitude value processed by the LNA. However, because of the characteristics of the loading formed by the inductor and the capacitor connected in parallel, an input signal with a frequency deviated from 2.43 GHz is attenuated via the LNA, and is completely filtered out if its frequency is located outside the allowed bandwidth 2.42–2.44 GHz. Please note that the combination of the inductor and the capacitor determines the center frequency and the operating bandwidth associated with the frequency response 10 shown in FIG. 1.
However, this kind of LNA is not applicable in a wideband communication system especially when the wideband communication system uses a high-frequency carrier to transfer data. Concerning an ultra-wideband (UWB) communication system, it requires an LNA having a great bandwidth from 10 GHz to 66 GHz. Therefore, the combination of the inductor and the capacitor according to the prior art narrow-band LNA is unable to generate a frequency response with such a great bandwidth. However, if a prior art wide-band LNA is adopted, much of the undesired noise could not be successfully suppressed through a single filtering bandwidth, and is injected into the ultra-wideband communication system, which degrades the signal quality.